Power reserve is the time during which a movement continues to function after the last energy input, without being worn or manually wound. It's not a marketing figure: it's a real mechanical measurement, linked to the barrel's capacity and the efficiency of the entire gear train. On a manual-wind watch, knowing when the barrel is nearly depleted is operationally necessary. On an automatic worn regularly, the declared reserve remains a reference point, not a daily promise.
The number of barrels is the first parameter. A single barrel rarely gives more than 42-48 hours on a standard movement: the Miyota 9039 stops at 42 hours, the RW4100 at 45. Two barrels in parallel or series change the situation: Chopard's L.U.C. 96.09-L caliber reaches 65 hours with the Twin-Barrel system, the Blancpain 1315 achieves 120 hours with three barrels. Four barrels stacked, as in the L.U.C. Quattro, bring it to 192 hours. High numbers don't automatically mean superior quality: they mean design choices with compromises in thickness, torque and regulation.
The efficiency of the gear train and anchor matters as much as the number of barrels. Rolex's Chronergy escapement, made of nickel-phosphorus, reduces friction losses in the 3285 caliber and contributes to the 70-hour autonomy without increasing barrel volume. Grand Seiko's Spring Drive UFA achieves 5 days with a compact dual barrel because the tri-synchro brake dissipates less energy than the traditional anchor. The platinum micro-rotor in the Czapek SXH7.1 maintains 60 hours in a 4.2 mm thick movement: platinum's density compensates for the rotor's reduced mass.
Power reserve indication is a chapter unto itself. The arc hand is the most common and most readable solution. There are also rotating dial indicators, as in the Longines Conquest Heritage Central Power Reserve with its 72 hours on a dual dial; digital indicators with portholes on the caseback, as in the JLC 874 caliber of the Reverso 8 Days; linear rack scales, as on the Hublot MP-17 with 240 hours; and ceramic microsphere systems, as in the Ressence Type 11. The Breva Segreto di Lario uses two separate retrograde hands — one for the first six days, one for the last 24 hours — to provide granular reading where precision really matters.
A detail that technical specifications almost always ignore: the non-linear indicator. On many movements the hand slows down as the spring discharges, because torque drops and consumption rate changes. A linear indicator in that context lies. When I evaluate a caliber on the bench, I look to see if the indicator is calibrated to the actual discharge curve or if it's merely decorative.